Many circuits are commonly used for electronic devices wherein it is important that a rapid transition be made from one voltage level to another. For example, output circuits are often used for converting from STL levels of internal logic to TTL levels necessary on circuit output pins. A typical output circuit for a TTL gate might normally have a low voltage level input and a high voltage output. If a high voltage input occurs, it is important that the output circuit provide a very fast transition to a low output. It is also important that such output circuits be able to drive high capacitive loads.
Previously developed output circuits for TTL gates have been developed with "speed up" elements which apply extra speed up current to an output transistor during output transition. However, such prior speed up circuits do not apply speed up current for a sufficiently long time or of a sufficiently high magnitude. Moreover, previously developed speed up circuits have been provided with a fixed threshold voltage for termination of speed up current. A need has thus arisen for a speed up output circuit which not only supplies a speed up current for a substantial period of time in order to provide rapid output transition, but which also allows easy variability of the length of time the turn off current is applied.